1. Field of the Invention
This invention relates to compensation circuits for lasers for correcting distortion in the output of such lasers, and in particular to an analog compensation system for a high power linear laser.
2. Description of the Relevant Art
The use of lasers for transmission of information has grown enormously in recent years. As fiber optic technology and laser technology improve, numerous applications are being found for communication over fiber optic cables providing enormous benefits with respect to conventional communication systems employing electrically conductive media. One problem with driving optical information over fiber optic cables, however, is distortion which is induced when the laser is driven at a high power. Many lasers are known to be linear over a small portion of their range; however, no lasers are linear over wide portions of their output power range. Additionally, in many applications it is desirable to acquire the least expensive laser and drive it at the highest possible power levels to obtain the maximum optical signal over the fiber optic cable. Such operations inevitably introduce distortion.
As a result, several techniques have been developed for correcting for errors and/or reducing distortion in fiber optic systems driven by lasers. For example, Muka, et al., in U.S. Pat. No. 4,672,192, describe noise reducing apparatus for a laser beam system. Muka, et al., teach a laser beam noise reduction apparatus which includes an acoustooptic cell. The cell receives a noisy input laser light beam and produces an undiffracted and a diffracted beam in response to a signal. The undiffracted beam is applied to a device which produces an information modulated output beam at a given image zone. The beam at the output of the acoustooptic cell is sampled and the information is used to reduce noise in the beam. Unfortunately, the Muka, et al., system is complicated, and requires diffraction gratings, partially silvered mirrors, and other expensive optical components.
Shibagaki, et al., in U.S. Pat. No. 4,733,398, describe a circuit for correcting for errors in a semiconductor laser. According to Shibagaki, et al., a drive circuit provides a semiconductor laser with current pulses corresponding to an input pulse signal. At the same time, a monitoring photodiode produces a light detection signal indicative of the actual laser output level. The detection signal is supplied to a subtractor to which the pulse signal is also supplied. This subtractor detects the difference between the two signals and produces an error signal. An integrator then produces an average value signal to obtain a resulting control signal. The control signal is used to modulate the signals for driving the laser to thereby maintain its output level at a constant level.
Straus, et al., in U.S. Pat. No. 4,075,474, teach an optical transmitter which utilizes two matched light emitting diodes to achieve distortion reduction. Straus, et al., achieve improved performance by modulating an electrical-to-optical converter using a modified signal. The modified signal is obtained using a matched emitting device which is used to drive an optical receiver. Errors in the received signal compared to the driving signal are then subtracted from the signal used to control the optical driver employed for the optical fiber. While the device described by Straus, et al., is suitable for some applications, it requires matched LEDs and thus is undesirable for high power applications.
Straus, et al., in "Linearized Transmitters for Analog Fiber Links," Laser Focus, Oct. 1978, pp. 54-61, describe an optical feedforward system in which the input signal is used to drive an amplifier for the LED. The LED output is monitored and supplied to an error control. The error control also receives the original input signal after an appropriate delay, then compares the two and uses them to drive a correcting LED.
A significant disadvantage of the approach employed by Straus, et al., is that the LEDs must be carefully matched. The entire system is predicated on the accuracy of the correcting LED matching the driving LED. An additional disadvantage is that because Straus, et al., do not tap the fiber to detect the signal on it, the errors corrected cannot match the actual signal as closely as is otherwise desired. A further disadvantage is that Straus, et al., monitors the driving LED by monitoring light which misses the fiber. As the driving signal changes, the LED output beam will widen and have a nonuniform power density. Thus, measuring the LED output outside the end of the fiber does not accurately describe the nature of the signal present within the fiber.